Table of Contents
This EIP defines Ethereum Node Records, an open format for p2p connectivity information.
Ethereum nodes discover each other through the node discovery protocol. The purpose of that protocol is relaying node identity public keys (on the secp256k1 curve), their IP address and two port numbers. No other information can be relayed.
This specification seeks to lift the restrictions of the discovery v4 protocol by defining a flexible format, the node record, for connectivity-related information. Node records can be relayed through a future version of the node discovery protocol. They can also be relayed through arbitrary other mechanisms such as DNS, ENS, a devp2p subprotocol, etc.
Node records improve cryptographic agility and handling of protocol upgrades. A record can contain information about arbitrary transport protocols and public key material associated with them.
Another goal of the new format is to provide authoritative updates of connectivity information. If a node changes its endpoint and publishes a new record, other nodes should be able to determine which record is newer.
The components of a node record are:
signature: cryptographic signature of record contents
seq: The sequence number, a 64-bit unsigned integer. Nodes should increase the number whenever the record changes and republish the record.
- The remainder of the record consists of arbitrary key/value pairs
A record’s signature is made and validated according to an identity scheme. The identity scheme is also responsible for deriving a node’s address in the DHT.
The key/value pairs must be sorted by key and must be unique, i.e. any key may be present only once. The keys can technically be any byte sequence, but ASCII text is preferred. Key names in the table below have pre-defined meaning.
||name of identity scheme, e.g. “v4”|
||compressed secp256k1 public key, 33 bytes|
||IPv4 address, 4 bytes|
||TCP port, big endian integer|
||UDP port, big endian integer|
||IPv6 address, 16 bytes|
||IPv6-specific TCP port, big endian integer|
||IPv6-specific UDP port, big endian integer|
All keys except
id are optional, including IP addresses and ports. A record without
endpoint information is still valid as long as its signature is valid. If no
udp6 port is provided, the
udp port applies to both IP addresses. Declaring
the same port number in both
udp6 should be avoided but doesn’t
render the record invalid.
The canonical encoding of a node record is an RLP list of
[signature, seq, k, v, ...].
The maximum encoded size of a node record is 300 bytes. Implementations should reject
records larger than this size.
Records are signed and encoded as follows:
content = [seq, k, v, ...] signature = sign(content) record = [signature, seq, k, v, ...]
The textual form of a node record is the base64 encoding of its RLP representation,
enr:. Implementations should use the URL-safe base64 alphabet
and omit padding characters.
This specification defines a single identity scheme to be used as the default until other schemes are defined by further EIPs. The “v4” scheme is backwards-compatible with the cryptosystem used by Node Discovery v4.
- To sign record
contentwith this scheme, apply the keccak256 hash function (as used by the EVM) to
content, then create a signature of the hash. The resulting 64-byte signature is encoded as the concatenation of the
ssignature values (the recovery ID
- To verify a record, check that the signature was made by the public key in the “secp256k1” key/value pair of the record.
- To derive a node address, take the keccak256 hash of the uncompressed public key.
The format is meant to suit future needs in two ways:
- Adding new key/value pairs: This is always possible and doesn’t require implementation consensus. Existing clients will accept any key/value pairs regardless of whether they can interpret their content.
- Adding identity schemes: these need implementation consensus because the network won’t accept the signature otherwise. To introduce a new identity scheme, propose an EIP and get it implemented. The scheme can be used as soon as most clients accept it.
The size of a record is limited because records are relayed frequently and may be included in size-constrained protocols such as DNS. A record containing a IPv4 address, when signed using the “v4” scheme occupies roughly 120 bytes, leaving plenty of room for additional metadata.
You might wonder about the need for so many pre-defined keys related to IP addresses and ports. This need arises because residential and mobile network setups often put IPv4 behind NAT while IPv6 traffic—if supported—is directly routed to the same host. Declaring both address types ensures a node is reachable from IPv4-only locations and those supporting both protocols.
This is an example record containing the IPv4 address
127.0.0.1 and UDP port
The node ID is
The record is signed using the “v4” identity scheme using sequence number
1 and this
The RLP structure of the record is:
[ 7098ad865b00a582051940cb9cf36836572411a47278783077011599ed5cd16b76f2635f4e234738f30813a89eb9137e3e3df5266e3a1f11df72ecf1145ccb9c, 01, "id", "v4", "ip", 7f000001, "secp256k1", 03ca634cae0d49acb401d8a4c6b6fe8c55b70d115bf400769cc1400f3258cd3138, "udp", 765f, ]
Copyright and related rights waived via CC0.
Please cite this document as:
Felix Lange, "EIP-778: Ethereum Node Records (ENR)," Ethereum Improvement Proposals, no. 778, November 2017. [Online serial]. Available: https://eips.ethereum.org/EIPS/eip-778.